Quantitative research of the origin of sulfur isotopes is a difficult problem that has puzzled geochemists all along. In the study of the middle and lower reaches of the Yangtze River and the Dongpo orefield in Hunan Province, the authors successfully applied the mathematical model of mixed population screening to quantitatively resolving the problem on the origin of sulfur isotopes, which is significant in finding out the source of mineralizing matter and metallogenic mechanisms.